1. Field of the Invention
The present invention relates to an improved composite channel field effect transistor and method of fabrication. More particularly, the invention relates to a composite channel field effect transistor which is less susceptible to failure due to threshold voltage shifts caused by hot electron effect when the device is operated at a high source to drain voltage level especially when composite layers of SiO.sub.2 --Si.sub.3 N.sub.4 are employed as the insulated gate dielectric.
2. Description of the Prior Art
Various enhancement mode and depletion mode insulated gate field effect transistor designs are known in the art, and the electrical characteristics of such devices are well known. For example, U.S. Pat. No. 3,745,425 to Beale et al describes a typical insulated gate field effect semiconductor device generally consisting of a monocrystalline semiconductor body of high bulk resistivity of one conductivity type having two low resistivity surface regions of the other conductivity type spaced apart in the body and forming two rectifying junctions with the bulk region of the body. A conductive layer is formed under a dielectric layer located on top of the surface of the body, the conductive layer extending between the two surface regions. Ohmic contacts are made to the low resistivity surface regions and the conductive layer. The dielectric layer is usually produced by oxidation of the semiconductor body.
Insulated gate field effect transistors fabricated as described above, can be operated in either depletion mode or enhancement mode in well known fashion. In either case, when the devices are operated at a source to drain voltage of relatively high magnitude, such as for example, larger than three to four volts, it has been recognized that hot electrons (hot electrons in an n-channel configuration and hot holes in p-channel configurations) are produced in the depletion region of the drain electrode which may possibly penetrate the silicon dioxide layer after redirection due to collision. A high incidence of this phonomena known as the hot electron effect can cause failure in the operation of the device due to threshold voltage shift.
Accordingly, a need exists in the prior art for an improved field effect transistor which is not subject to threshold voltage shift when operated at relatively high source to drain voltage levels.